1. Field of the Invention
The present invention relates to a molded transformer which is used in various types of video apparatus, household electric appliances, acoustic apparatus, industrial apparatus or communication apparatus.
2. Description of the Related Art
A conventional transformer will be described below.
FIG. 11 shows the internal structure of a conventional transformer. A spool 1 has a central through-hole 2. The spool 1 also has a collar 8 at each of the two ends thereof, and a lower collar 4 at the lower end of each of the collars 8. Maker terminals 5 for wiring and user terminals 6 used by the user are formed integrally with the spool 1. A conductive wire 3, such as a copper wire, is wound around the spool 1 and is connected to the maker terminals 5 to form a coil bobbin. An EE type or EI type ferrite core 7 inserted into the coil bobbin 1 is fixed to the coil bobbin 1 by coating an adhesive 14, such as an epoxy resin, on the joining surface of the ferrite core 7 and then by heat setting the adhesive 14 at a temperature ranging from 100.degree. C. to 200.degree. C. to form a transformer body in which a closed magnetic path is formed. The thus-arranged transformer body is inserted into a casing 9 made of a plastic resin, a highly insulated resin 10, which may be silicon resin, is injected into the casing 9, and then the casing 9 is sealed by a bottom plate 11, which may be made of a plastic resin, in such a manner that the user terminals 6 are exposed through the resin to form a transformer.
FIG. 12 shows a shielded type transformer in which a box-shaped shielding casing 12 covers the transformer arranged in the manner described above from above.
In the aforementioned conventional transformer, the number of parts is large, and the production process is complicated. The plant and equipment investment required for automating the wire winding process and the core assembly process, which are necessary for the function of the transformer, increases the amount which has to be invested but this investment has a beneficial effect. The plant and equipment investment required for automating the subsequent processes, such as the casing insertion process, the silicon injection process and the bottom plate insertion process, which are necessary for the additional functions, increases the amount which has to be invested too much.
Furthermore, since the conventional transformer is manufactured by combining and fitting the bare transformer body, the casing and the bottom plate, the shape of the transformer and the dimensions of the user terminals are very unstable. This deteriorates the yield of the transformer automatic mounting process performed by the user.
A reduction in the number of parts and an improvement in the shape of the transformer and in the dimensional accuracy of the user terminals may be achieved by plastic molding the entire transformer. Generally, plastic molding employs thermosetting resins, such as an epoxy resin. The thermosetting resins require a long molding time, and generate burrs during molding. Therefore, a burr removing process must be conducted after molding, and the productivity of the molding process is thus low.
In the conventional transformer, when a transmission noise, which is an externally radiant noise induced in the transformer and transmitted to the circuit, is generated or when a malfunction of the circuit occurs due to leaking magnetic flux generated from the transformer, the transformer body is covered with the box-shaped shielding casing 12 to suppress, the influence of the noise. However, this increases assembly manhours. Also, when the user desires to mount the parts with a high density, insulation between the adjoining parts must be provided. Therefore, the transformer mounting position is limited, thus limiting the high-density mounting of the parts.
Furthermore, since the joining surface of the EE type or EI type ferrite core 7 is fixed using the adhesive 14 in the core assembly process, the adhesive 14 coating and setting processes are required, making the production process complicated. Also, since an epoxy type thermosetting adhesive having a high mechanical strength and a high heat resistance is generally employed to adhere the ferrite core 7, setting of the adhesive 14 requires a setting tank of a temperature ranging from 100 to 200.degree. C., thus increasing the amount which has to be invested in plant and equipment.